1. Field of the Invention
The present invention relates to measuring a shape. More particularly, the measuring is performed in a borehole penetrating the earth.
2. Description of the Related Art
In exploration and production of hydrocarbons, it is often necessary to drill a borehole into the earth to gain access to the hydrocarbons. Equipment and structures, such as borehole casings for example, are generally disposed into a borehole as part of the exploration and production. Unfortunately, the environment presented deep into the borehole can place extreme demands upon the equipment and structures disposed therein. For example, the equipment and structures can be exposed to high temperatures, pressures, and forces that can deform their shape and, thus, their operation and longevity.
In order to monitor the health of the components disposed downhole, a conventional distributed strain sensing system (DSSS) may be used. DSSS sensors may be attached to a component at various locations usually at different depths in the borehole. The sensors can measure the strain imposed upon the component. From the strain measurements, it can be determined if the component is being deformed.
In one class of DSSSs known as optical reflectometry systems, a linear series of fiber Bragg gratings is etched into an optical fiber. Each fiber Bragg grating (FBG) in the series acts as a strain sensor. The optical fiber, in one example, is continuously and rigidly attached to a borehole casing and wrapped around the casing for a certain longitudinal length. Because the optical fiber is continuously and rigidly attached to the casing, any strain experienced by the casing will also be experienced by the optical fiber. As each FBG is exposed to a strain, the dimensions and, thus, the optical characteristics of each FBG will also change in relation to the strain. For example, a tension experienced by the casing will be experienced by the FBG. Conversely, a compression experienced by the casing will also be experienced by the FBG. The strain is typically measured from baseline data obtained with the component not experiencing a strain. A beam of light reflected from the optical fiber is used to interrogate each of the FBGs to obtain their strain measurements.
Operations for producing hydrocarbons can be very expensive. Hence, production operators strive to minimize the time is takes to deploy components downhole. Unfortunately, it can take a significant amount of time to continuously and rigidly attach the optical fiber in the conventional DSSS to the component to be deployed downhole. In one example, the optical fiber is secured to the component with epoxy, which takes significant time to apply and cure.
Therefore, what are needed are techniques to determine a change in shape or deformation of a component, particularly, if the component is going to be deployed downhole.